Friction is one of the most heavily tested contact forces on the AP Physics 1 exam, and it is the topic where otherwise strong candidates lose the most rubric points for the wrong reason: they pick the right direction but the wrong coefficient. The exam treats kinetic friction (μk) and static friction (μs) as two different physical objects with two different equations, two different numerical values, and two different rubric rows. A correct final number built on the wrong equation still loses the points. This article walks through the friction FRQ the way the actual scoring guide reads it: the setup line, the free-body diagram, the inequality versus the equality, the impending-motion row, and the unit row that quietly decides between a 3 and a 4 on the question.
Why AP Physics 1 treats kinetic and static friction as separate quantities
Physics 1 spends a surprising amount of its friction section not on the formula f = μN but on the rule that which μ belongs in that equation depends on the state of motion between the two surfaces. Most candidates can write the equation. The exam is testing whether the candidate can look at a scenario and decide whether the surfaces are sliding across each other or stuck together. That decision is the single highest-leverage judgement on a friction question, because every other line of work — the normal force, the acceleration, the sign of the net force — flows from it.
The College Board framework is explicit. Kinetic friction is the resistive force that appears once two surfaces are sliding past one another, and its magnitude is given by fk = μkN, where N is the magnitude of the normal force pressing the surfaces together. Static friction is the resistive force that prevents sliding from starting, and its magnitude is bounded by fs ≤ μsN. The static case is an inequality until the moment motion is about to begin, at which point the static friction reaches its maximum value, fs,max = μsN, and any additional applied force will produce motion. Candidates who treat static friction as a fixed value rather than a self-adjusting response get the setup wrong, and the rubric scores that mistake directly.
For most candidates reading this, the practical implication is that the friction FRQ has two distinct setup paths and a third transition path. Path one is a block sliding at constant speed or accelerating: kinetic friction applies, the equation is an equality, and the work is finding N. Path two is a block sitting still or moving at constant velocity with a horizontal push: static friction applies, the equation is an inequality, and the work is bounding μs from data. Path three is the impending-motion row, which is the bridge between the two and the most commonly mis-scored line on the question.
In my experience the candidate who has practised the impending-motion line out loud three or four times gains roughly a full point on the friction FRQ, because the rubric awards that row for stating both the equality fs = μsN and the condition that this is the maximum value. A candidate who only writes the equality, or only writes the inequality, loses the row even if every number that follows is correct.
The five friction FRQ shapes the rubric actually scores
After a few years of reading scoring guides and student work side by side, the friction FRQ tends to arrive in one of five shapes, and the rubric has a recognisable setup line for each. Recognising the shape before reading the data section is worth more than memorising any single formula, because it tells the candidate which μ to reach for before the timer is ticking.
Shape one: a block on a horizontal surface with a known horizontal push and a stated constant velocity. The setup line is fk = μkN with the implicit second row that the net horizontal force is zero. The work is then to identify N as the weight on a horizontal surface, substitute, and solve for μk. Rubric row 1: correct equation. Row 2: correct substitution of N. Row 3: numerical answer with units.
Shape two: a block on a horizontal surface being pushed but not moving. The setup line is fs ≤ μsN, and the question is asking the candidate to bound μs from the data given. The rubric scores the inequality form, not an equality, and a common error is to write fs = μsN even though the block has not yet started to slide. That equality is only valid at impending motion, not before.
Shape three: a block on an incline with or without an applied force. Here the normal force is no longer equal to mg, and the work expands. N = mg cos θ, and the friction component along the plane is μN, with the sign of the friction force opposing the direction of motion or the direction of the impending motion. The rubric row that students most often miss on this shape is the N row, because they assume N = mg and lose every subsequent point that depends on the corrected normal force.
Shape four: two blocks in contact, with a force applied to one, and the question of which block — if any — slips relative to the other. This is the impending-motion shape in disguise. The setup line is to treat the two blocks as a single system, find the common acceleration, then compute the static friction required to produce that acceleration on the trailing block. The rubric scores the comparison: is the required static friction less than μsN? If yes, they move together. If no, the trailing block slips and the front block accelerates under kinetic friction. The transition between the two answers is the heart of the scoring.
Shape five: a block on a flat surface with a gradually increasing horizontal push, plotting friction versus applied force. The graph rises linearly with slope one while the block is stationary, plateaus at fs,max = μsN at the moment of impending motion, then drops to a lower constant value fk = μkN once the block is sliding. The rubric scores the kink (impending motion), the height of the plateau (μsN), and the lower horizontal line (μkN). The shape of the graph is the answer; numbers are decoration.
| FRQ shape | Which μ to use | Setup equation | Rubric row most often lost |
|---|---|---|---|
| Sliding block, constant velocity | μk | fk = μkN, Fnet = 0 | Writing f = μN as a generic line |
| Pushed block, not moving | μs | fs ≤ μsN | Writing fs = μsN before motion |
| Block on incline | Both, depending on state | N = mg cos θ, f = μN along plane | The N = mg cos θ row |
| Two blocks, impending slip | μs first, μk after slip | a = F/(m1+m2), then compare | The comparison frequired < μsN row |
| Friction versus applied force graph | μs on plateau, μk after drop | Plateau at μsN, drop to μkN | Drawing the kink, not just the numbers |
How the rubric scores the setup line on a kinetic-friction question
The kinetic-friction setup line is a deceptive two-line piece of work. The first line identifies the equation fk = μkN and the second line identifies the normal force N, and the rubric treats these as two separate rows even though they look like one line to most candidates. A student who writes 'the friction force is μmg' on a horizontal surface is implicitly assuming N = mg, which is true on a horizontal surface but the rubric still wants to see that substitution written out. On an inclined surface, where N is not mg, the implicit assumption becomes a numerical error rather than a notational one, and the rubric will deduct for the missing cos θ.
The second row the kinetic-friction rubric scores is the direction of the friction force. Kinetic friction opposes the relative motion of the surfaces, which on the FRQ is usually a clear statement because the question gives the block a known velocity. The rubric's tolerance here is high — a small arrow on the free-body diagram pointing opposite to the velocity is enough — but the score is real, and a free-body diagram with friction drawn in the same direction as the applied force loses the point.
The third row is the numerical answer with units, and the units matter. A candidate who writes μ = 0.32 with no unit is fine for μ, which is dimensionless, but a candidate who writes f = 3.2 without N for newtons has lost a row even if 3.2 is the right number. For most candidates this is a habit issue rather than a knowledge issue, and the fix is to write the unit on every line, even the intermediate ones.
The fourth row, present on longer FRQs, is the justification row. The justification is usually a one-sentence argument for why kinetic friction applies rather than static, and it can take one of two forms: a direct statement that the block is sliding, or an indirect statement that the relative velocity between the surfaces is non-zero. Either is acceptable, and either scores the row. A candidate who writes 'because of friction' has not justified anything and loses the row.
How the rubric scores the setup line on a static-friction question
Static friction is the harder of the two to set up because the equation is an inequality, and many candidates refuse to write the inequality. They rewrite it as an equality, which is a form of wishful thinking: the candidate wants the problem to be in the kinetic regime, where the algebra is cleaner, and they force the setup into that shape. The rubric is patient with this mistake. It scores the inequality form on one row and the bound on the next, and a candidate who writes the equality from the first line loses both rows.
The second row the static-friction rubric scores is the resolution of forces perpendicular to the surface. On a horizontal surface this is a single line: N = mg (or N equals the vertical component of any other applied force). On an incline the same row takes two lines: N = mg cos θ on the perpendicular axis and mg sin θ as the component of gravity along the plane. A candidate who skips the perpendicular resolution and writes N = mg on an incline loses a point that the rest of the question quietly depends on.
The third row is the bounding statement: the candidate is expected to write that the static friction is at most μsN, and then to use the data in the problem to bound μs from above, from below, or both. The exact form of the bound depends on what the question asks. A common variant gives a range of applied forces and asks for the range of μs consistent with the block remaining stationary. The rubric scores the candidate's ability to invert the inequality correctly: a larger applied force requires a larger μs, not a smaller one, and the candidate who flips the sign of the inequality loses the row.
The fourth row, when the question is asking about impending motion, is the equality fs = μsN with the additional statement that this is the maximum value. This is the impending-motion row, and it is the row most often missed. The candidate writes the equality because they recognise it from the kinetic case and reuses it, but they forget the qualifier. The qualifier matters because the rubric uses it to decide which side of the question the candidate is on: a candidate who has stated impending motion is on the static-friction side until the moment of slip, and a candidate who has not is treated as if they have not bounded μs at all.
Impending motion: the row that flips between static and kinetic
Impending motion is the most clinically important concept in the friction section, and it is also the concept most often taught as a single sentence rather than as a worked step. The single sentence is correct: impending motion is the moment at which static friction has reached its maximum value and any additional applied force will produce motion. The worked step is what the rubric actually scores, and the worked step has three components.
First, the candidate must state that static friction is at its maximum: fs = μsN, with the word 'maximum' or its equivalent present. Second, the candidate must state the condition that defines this moment, which is usually a comparison between the applied force or the gravitational component along the plane and the maximum static friction. Third, the candidate must connect the moment to the kinetic case by writing that for any force larger than this threshold, the friction drops to fk = μkN. The three components together form a single rubric row, and a candidate who writes two of the three loses the row in the same way a candidate who writes zero of the three loses it.
In practice the most common single-component loss is the third one. The candidate sets up the impending-motion equality, solves for the threshold force or the threshold angle, and stops. They never say what happens after the threshold, and the question — which usually asks for the motion after the threshold — goes unanswered. The fix is mechanical: after solving for the threshold, the candidate writes a single line, 'for F greater than Fthreshold, the block accelerates with kinetic friction fk = μkN', and the third component of the row is satisfied.
Impending motion is also the place where the exam introduces a coefficient inequality that is worth committing to memory. For most real surfaces, μs > μk: the static coefficient is larger than the kinetic coefficient, which is why a block that has been sitting still takes a slightly larger force to start moving than the force required to keep it moving. The exam does not require this inequality in a vacuum, but it scores it on the graph-shape question: a candidate who draws the post-slip kinetic line above the impending-motion plateau is contradicting the standard inequality and loses the graph row.
Common pitfalls and how to avoid them
The friction section has four recurring errors, and three of them are scoring-guide-visible. The first is the equation-mix error, where the candidate writes f = μN and uses the right form of the equation but the wrong μ. They see the block moving and use μs, or they see the block stationary and use μk. The scoring guide treats this as a one-row error rather than a one-line error, because the rubric's row for 'which μ' is independent of the row for 'which equation'. A candidate who writes the wrong μ with the right equation loses a single row rather than two, but they still lose it.
The second recurring error is the sign-flip error on an incline. A block sliding down an incline has kinetic friction acting up the plane, and a block being pushed up an incline has kinetic friction acting down the plane. A candidate who draws friction in the wrong direction on a free-body diagram for an inclined question typically loses the acceleration row on the rubric, because the acceleration comes out with the wrong sign. The fix is to draw the velocity vector first and the friction vector second, opposite to the velocity, and to do this on the diagram rather than in the algebra.
The third recurring error is the missing-impending-motion error, described above. The fourth, less common but real, is the unit-on-N error, where the candidate computes a normal force in newtons and then plugs it into f = μN using a μ with implicit units, producing a friction force that is dimensionally wrong. The fix is the same as for the unit-on-friction answer: write the unit on every line, and check that the units on both sides of the equation match.
For most candidates the highest-leverage single habit is to write, on every friction FRQ, the explicit two-line statement 'kinetic friction applies because the surfaces are sliding' or 'static friction applies because the surfaces are not sliding'. The habit takes fifteen seconds and scores a row that is otherwise easy to lose to an unclear setup.
Multiple-choice traps: the four sentence patterns that flip the coefficient
On the multiple-choice section, friction questions rarely test the formula directly. They test the candidate's ability to read a sentence and choose the right coefficient for the situation the sentence describes. Four sentence patterns show up repeatedly, and each one has a characteristic trap.
Pattern one: 'a block slides across a horizontal surface at constant speed'. The trap is the constant-speed phrase, which sounds like an equilibrium and tempts the candidate into the static-friction equation. The correct reading is that the block is sliding, so kinetic friction applies, and the constant speed is a way of giving the candidate the data to solve for μk. The sentence is an equality setup, not a static-friction setup.
Pattern two: 'a box sits on the floor of a truck that is accelerating, and the box does not slide'. The trap is the word 'sits', which the candidate reads as 'is in static equilibrium'. The correct reading is that the box is in static equilibrium in the truck frame but accelerating in the ground frame, and static friction is providing the forward force. The candidate who reads 'sits' as 'no net force' gets the problem wrong; the candidate who reads 'sits on the truck floor' as 'no relative motion with the truck' gets it right.
Pattern three: 'a block is placed on a horizontal surface and a force is gradually increased from zero until the block begins to move'. The trap is the 'gradually increased' phrase, which the candidate reads as a kinematics problem. The correct reading is a friction-versus-force graph, with the static-friction plateau and the kinetic-friction drop. The question is usually asking for the coefficient ratio, not the acceleration, and the candidate who tries to compute a kinematics answer has misread the question.
Pattern four: 'two blocks are stacked, and the lower block is pulled'. The trap is the stack, which the candidate reads as a single-system problem. The correct reading depends on whether the upper block slips relative to the lower block, and the rubric is testing the candidate's ability to make that distinction. The decision is made by the impending-motion row, and the candidate who skips that row gets both the upper-block answer and the lower-block answer wrong.
Preparation strategy: how to build friction fluency before exam day
Friction fluency is built by practising the setup line out loud before practising the algebra, and the right preparation order is the opposite of what most students do. Most students read a friction problem, jump to the formula f = μN, and then realise partway through that they do not know which μ to use. The fix is to spend the first thirty seconds of every practice problem writing the two-line 'which μ' statement before reaching for the equation.
The next preparation step is to practise the impending-motion row in isolation. A short drill is to take any static-friction problem, find the threshold value, and then write the one-sentence statement of what happens after the threshold. After a dozen such drills, the impending-motion row becomes a habit and stops being a separate step.
The third preparation step is to draw free-body diagrams with the velocity vector first, the friction vector second, and the rest of the forces third. The order matters because the velocity vector is the only unambiguous direction in the diagram, and the friction vector is determined by it. Drawing the friction vector first is a recipe for sign errors.
For students preparing in the eight weeks before the exam, the highest-leverage use of practice time is to take three released FRQs from previous years, work them under timed conditions, then score them against the published scoring guide row by row rather than just for the final answer. The row-by-row scoring surfaces the impending-motion row, the direction row, and the unit row as separate items, and the candidate sees exactly which rows are giving away points.
The exam awards up to 5 points on the friction FRQ for a complete answer, and the breakdown tends to be 1 point for the equation, 1 point for the normal force, 1 point for the impending-motion or direction row, 1 point for the numerical answer, and 1 point for the justification. A candidate who scores 3 or 4 on the question is usually losing the impending-motion row and the justification row, and both are addressable in a single preparation session.
Tying it back to the exam format and scoring
AP Physics 1 is a three-hour exam with two sections: a 90-minute multiple-choice section of 50 questions and a 90-minute free-response section of five questions, one of which is a laboratory-based question and four of which are conceptual or quantitative short-answer questions. Friction appears in both sections. On the multiple-choice side it is usually tested as a 2-to-3 question cluster, with one question on the static case, one on the kinetic case, and one on the impending-motion transition. On the free-response side it is the dominant force on roughly one FRQ per year, often combined with a Newton's-second-law problem or an inclined-plane problem.
The composite score is on a 1-to-5 scale, with 5 representing the top decile of college-ready performance. To earn a 5, the candidate needs to combine correct setup lines with correct algebra, and the friction section is one of the places where the two are most easily separated. A candidate with strong algebra and weak setup language tends to score 3 or 4 on the friction FRQ; a candidate with strong setup language and weak algebra tends to score 3 or 4 as well. The 5 belongs to the candidate who has both.
The exam is scored on a curve, and the friction question is one of the higher-discrimination items on the test, which means it separates strong candidates from weak ones more sharply than most other questions. The implication for preparation is that time spent on the friction setup line is among the highest-leverage uses of preparation time available, because a small improvement in setup fluency produces a disproportionate gain in the final score.
Conclusion and next steps
Static and kinetic friction are not two versions of the same equation; they are two different physical objects with two different rubric rows, and the exam is testing the candidate's ability to tell them apart at a glance. The setup line — which μ, which equation, which direction, which inequality — is where the points are won, and the impending-motion row is the single most valuable line on the question. With roughly eight weeks of preparation, a candidate who practises the setup line out loud, drills the impending-motion row in isolation, and scores practice FRQs row by row against the published scoring guide can move from a 3 to a 5 on the friction FRQ without any new physics, just by writing the rubric's rows in the order the rubric wants them.
AP Courses' AP Physics 1 programme pairs each student with a tutor who scores friction FRQs row by row against the released scoring guide, drills the impending-motion setup line until it is automatic, and turns a target 5 into a concrete preparation plan built around the candidate's specific row losses.